The present invention relates to a method for determining the attitude error of a space vehicle by means of star acquisition and star identification.
The attitude of a space vehicle, particularly a satellite, relative to a spatial system of coordinates must always be precisely known in order for the space vehicle to fully carry out its mission. This is so, for example, when antenna systems, telescopes or star sensors mounted on the satellite must be precisely aligned with specific target areas. Moreover, maneuvers are frequently required in order to carry out a change of alignment or orientation of the satellite in the spatial coordinate systems. Rotational movements of this type may be carried out by means of nozzle systems mounted on the space vehicle, by changing the rotational speed of on board flywheels, or by the interaction of magnetic moments generated in the satellite with exterior magnetic fields. The resulting rotational angles about the three axes of the system of coordinates fixed on the satellite may be measured, for example, by means of high-precision gyroscopes. Thus, based on a precisely known starting orientation, the desired new attitude can be achieved through precise measurement of rotational movements of the space vehicle. However, this determination of the attitude is always characterized by certain errors which depend, in particular on the precision of the measuring instruments that are used.
Star sensors may be used in order to determine attitude errors of this type; that is, differences between the desired and actual new attitude. Such star sensors are conventionally mounted on the space vehicle, and usually have a lens system by means of which a relatively small areal segment of the celestial sphere is imaged on an areal photosensor arrangement, such as a two-dimensional CCD-array. The visual field of the star sensor is determined by the preferably rectangular or square photosensor arrangement in the focal plane of the lens system. This may, for example, be in the order of 5.degree..times.5.degree.. In those instances where at least the approximate alignment of the space vehicle is known, it will also be known which segment of the celestial sphere appears in the visual field of the star sensor, that is, which stars are imaged on the photosensor arrangement. It may therefore be predicted which of the brightest stars are to be expected at particular positions in the visual field or on the photosensor arrangement. A two-dimensional, that is, generally rectangular, system of coordinates may be assigned to the visual field or the surface of the photosensor arrangement, the origin of this system of coordinates preferably being in the center of the visual field. By comparing the sensed positions of known stars with their desired positions in the visual field of the star sensor, the attitude error relative to the desired new attitude of the space vehicle can be determined.
It is customary to select a certain number of fixed stars and to compile them into a catalogue of stars, the content of which may be determined by the mission of the space vehicle. The radiation spectrum of the catalogued stars must fall at least partially within the sensitivity range of the photosensor arrangement; furthermore, they must have a certain minimum brightness and maintain this brightness as constantly as possible. In addition, they must not change their position in the celestial sphere and must be so far away that they are imaged as a point. For a catalogue of stars of this type, a system of coordinates may be selected which is centered in the solar system; for example, in the solar center or in the earth center. The catalogue of stars will then contain the respective coordinates with respect to the spatial system of coordinates as well as the respective brightness of the star (magnitude). It is practical to select, if possible, only the brightest stars. However, this may also depend on which segments of the celestial sphere are to be expected in the visual field of the star sensor as a function of the special mission. These segments may differ considerably with respect to the stellar density.
A method of the above mentioned type, in which a star sensor and a catalogue of stars are used, and in which at least three catalogued stars are selected from the catalogue of stars (one being defined as the guide star), is disclosed in U.S. Pat. No. 4,680,718. In those instances in which the initial orientation of the satellite is unknown, or is known only very imprecisely, it cannot be predicted which stars will appear in the visual field of the star sensor. Stored in the catalogue of stars, in addition to data concerning the position of a number of reference stars, is certain information concerning different three-star combinations, the reference star in each case being part of the three-star combination. This information includes the brightness total of these three stars, as well as the surface of the triangle which is in each case defined by them. The same quantities are then determined for respective different combinations of three of the stars observed in the visual field of the star sensor, and are subsequently compared with those stored in the catalogue of stars. This method requires high expenditures with respect to storage space and computing operations as a result of the fact that the orientation of the satellite at first may be arbitrary, and therefore the whole informational content of the star sensor as well as of the stored catalogue of stars must be used.
German patent document DE-OS 14 48 564, also discloses a method of recognizing stellar images in space vehicles where the initial orientation of the satellite is almost unknown. For this purpose, star sensors having very large visual fields are used, for example, with diameters of approximately 44.degree.. Approximately two hundred stars, and as many stellar images, are stored. At least five stars must be visible in the visual field of the star sensor, depending on the orientation of the satellite, and a so-called "central star" is selected approximately in the center of the visual field of the star sensor. The distances between this central star and the other visible stars are then determined and stellar constellations with the same distances are found in the catalogue of stars. If necessary, the angular distances and the brightness of the observed stars are also used. Here also, considerable expenditures are required during the analysis of the star sensor information because of the initially unknown, or only imprecisely known, orientation of the satellite. This method is therefore also not very suitable for a case in which the orientation of the satellite is known fairly accurately before the determination of the position.
When a very precise gyroscope is used for measuring the rotational movement after a star acquisition and identification the following steps are used to measure the catalogued stars appearing in the visual field of the star sensor: First, it is determined for each rotational motion, starting from an almost precisely known initial attitude, which catalogued stars are expected to appear at particular positions in the visual field of the star sensor following completion of the rotation. Expediently, two catalogued stars are selected, and a respective window is placed around their desired positions which corresponds dimensionally to twice the expected maximum attitude error. This window will generally be rectangular, and preferably square. In the case of high-precision gyroscopes and correspondingly low maximum attitude errors, this window is so small that, except for the selected catalogued star, no other star of comparable brightness will be present in it.
The existence of an attitude error will cause the catalogued star to be displaced relative to its expected position. From this difference between the measured actual and the expected position of the catalogued star, conclusions can be drawn concerning an attitude error of the space vehicle in two dimensions. In this case, the measurement may be performed in such a manner that the output signals of only the photosensors located within the selected window are used for the analysis. By mean of the comparison of the output signals of these individual sensors according to defined methods, the position of the respective brightest star can be easily determined within the window.
This simple-appearing method of operation, however, requires the use of high-precision (and therefore very expensive) measuring instruments, particularly gyroscopes, since the use of less precise measuring instruments will yield a larger attitude error, and the window placed around the desired position of the selected catalogued star must therefore be enlarged correspondingly in its dimensions s that this catalogued star will be reliably situated within the window. Moreover, starting from a certain relatively larger window size, it is unavoidable that, in addition to the selected catalogued star, other stars of comparable brightness also appear in the window. The selected catalogued star can then no longer be easily identified.
It is therefore an object of the present invention to provide a method of the generic type described above which, on the one hand, does not require the use of high-precision and therefore expensive measuring instruments for the determination of the rotational angle covered during the rotational movement and, on the other hand, can be carried out with minimal storage and computing expenditures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.